There is known as an optical disk memory a disk of the write-once type using a thin film of TeO.sub.x (0&lt;x&lt;2.0) mainly composed of Te and TeO.sub.2. A chalcogen material, Ge.sub.15 Te.sub.81 Sb.sub.2 S.sub.2, is also known as reported by S. R. Ovshinsky. This material is utilizable for recording information by heating and melting it by application of a laser beam and quenching for amorphization. The recorded information can be erased by heating by irradiation with a laser beam and subsequent gradual cooling for crystallization. Moreover, thin films made of combinations of chalcogens and the elements of the Group V or VI of the Periodic Table such as Ge, e.g. As.sub.2 S.sub.3, As.sub.2 Se.sub.3 or Sb.sub.2 S.sub.3, are widely known.
One of procedures of recording information on these thin films by means of a laser beam and erasing the information is described. A thin film is initially crystallized and a laser beam with about 1 .mu.m.phi. whose intensity is modulated according to information is applied, for example, to a recording disk having a thin film thereon under conditions of rotation of the disk. Portions where irradiated with the laser beam are heated to a temperature higher than the melting point of the thin film and then quickly cooled, thereby causing the information to be recorded as amorphous dots. For the erasure of the information, a spot beam which is elongated along the direction of rotating track or tracks of the disk is irradiated, so that the thin film is heated and is then recrystallized by the effect of gradual cooling with the elongated spot beam.
In recording mediums where information is recorded and erased through heating, temperature rise, melting, quenching and heating, temperature rise, gradual cooling, respectively, by laser beam irradiation of the thin film, the quality of signals may vary in correspondence with the heating cycle. The variation is considered to result from thermal, mechanical damages of a material for substrate which are produced by repetition of a number of cycles of rapid heating over 400.degree. C. by application of a recording spot beam and an erasing spot beam and cooling. In addition, the recording thin film may suffer thermal, mechanical damages. Depending on the composition, the recording thin film may undergo a local change in the composition, component of the film or so-called segregation.
A problem has been involved in that where the substrate or recording film undergoes such a change as mentioned above, an increasing number of noises generate and the cycle characteristic deteriorates during the recording, reproducing and erasing cycles.